1. Field of the Invention
The present invention relates to an underwater laser processing apparatus and an underwater laser processing method, and more particularly, to an apparatus and a method capable of repairing or welding cracks in a structure underwater using a laser beam for maintaining stability and safety thereof.
2. Description of the Related Art
Conventionally, cracks in a structure such as a nuclear reactor is processed by renewing the whole cracked part or reinforcing the part by fittings and the like. Using a yttrium aluminum garnet (YAG) laser has been recently researched to repair the part directly, which can be processed without discharging water from the nuclear reactor.
Such cracks must be repaired immediately, such as to remove the cracks completely by using a grinder and the like and then weld fillers on the cracks. In order to restrain the growth of cracks, it is important to isolate the cracks from the atmosphere. Sealing up the crack openings with fillers is one effective method.
Some conventional methods for repairing cracks by using a laser beam underwater are disclosed by Japanese patent disclosures PH9-10977 and PH10-180476. FIG. 1 is a sectional view showing an exemplary laser repairing apparatus.
In an optical head 1, an optical unit 3 having lenses 2 is secured. A laser beam 5 guided to the optical head 1 by an optical fiber 4 is condensed by the optical unit 3, and is irradiated from the top of the optical head 1 to the part to be welded in a work or workpiece 6 such as a pipe.
The top of the optical head 1 is formed as a thin nozzle 7 capable of approaching close to a laser spot. A welding wire 8 is provided to the laser spot by a wire feeding path 9, and shield gas 10 is supplied by a gas source (not shown) to the nozzle 7 by a gas feeding path 11.
The laser beam 5 oscillated by a YAG laser is guided to the optical head 1 disposed under water 12 via the optical fiber 4, and the laser beam 5 condensed by the optical unit 3 is irradiated to the workpiece 6. During the process, the welding wire 8 is supplied to the laser spot via the wire feeding path 9.
Meanwhile, the shield gas 10 is supplied to the nozzle 7, via the gas feeding path 11, and is injected from the top of the nozzle 7 to the laser spot. In this process, water is excluded from the point to be welded by the injected shield gas 10 and the gas atmosphere 13 allows the welding process to be generated. Note that the shield gas 10 can keep the dried condition of the welding wire 8.
However, to maintain the welding process the shield gas 10 has to be supplied continuously and water has to be excluded to form the gas atmosphere 13. Here, the optical head 1 is tapered at the top, and if a certain volume of bubbles 13a are created, the same amount of water around the nozzle 7 enters the optical head 1 easily. Thereby, it is difficult to maintain the gas atmosphere 13. When the optical head 1 is in a horizontal position, maintaining the gas atmosphere 13 is much more difficult and the above problem may be exacerbated.